Fixed wing aircraft with trailing rotors

ABSTRACT

A plurality of port-side tilt rotors and a plurality of starboard-side tilt rotors are configured to rotate between a first rotor position and a second rotor position. The port-side tilt rotors are coupled to a trailing edge of the fixed and forward-swept wing on the port side and the starboard-side tilt rotors are coupled to the trailing edge of the fixed and forward-swept wing on the starboard side. The aircraft also includes a fuselage. When the port-side and starboard-side tilt rotors are in the first rotor position, at least some of a lift to keep the aircraft airborne comes from thrust output by the rotors at least some of the time. When in the second rotor position, at least some of the lift to keep the aircraft airborne comes from aerodynamic lift on the fixed and forward-swept wing at least some of the time.

CROSS REFERENCE TO OTHER APPLICATIONS

This application is a continuation of co-pending U.S. patent application Ser. No. 15/902,281 entitled FIXED WING AIRCRAFT WITH TRAILING ROTORS filed Feb. 22, 2018 which is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

New types of aircraft which can take off and land with smaller footprints are being developed. For example, these aircraft may be used in areas where there is no airport or runway. Some such aircraft designs have a tilt wing (i.e., a rotatable wing) to which rotors are mounted. During the vertical takeoff, the tilt wing is in one position (e.g., with the rotors oriented to provide thrust downwards). Once the aircraft ascends to a sufficient or desired altitude, the tilt wing is rotated into another position (e.g., with the rotors oriented to rotate to provide thrust backwards) and the aircraft is able to fly more efficiently in a substantially forward mode or manner. To vertically land, the tilt wing is again rotated back into its original position. Although tilt wings can fly, new types of aircraft which (as an example) are more efficient (e.g., which reduces power consumption and/or increases flight range) and/or have higher maximum speeds would be desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are disclosed in the following detailed description and the accompanying drawings.

FIG. 1 is a diagram illustrating a top view of a forward swept, fixed wing multicopter embodiment with tilt rotors.

FIG. 2A is a diagram illustrating a bottom view of an embodiment of boundary layer thicknesses with the motors off.

FIG. 2B is a diagram illustrating a bottom view of an embodiment of boundary layer thicknesses with motors on.

FIG. 3A is a diagram illustrating an example of a tilt wing configuration with corresponding lift vector, thrust vector, and drag.

FIG. 3B is a diagram illustrating an example of a fixed wing configuration with a leading edge mounted tilt rotor and corresponding lift vector, thrust vector, and drag.

FIG. 3C is a diagram illustrating an embodiment of a fixed wing configuration with a trailing edge mounted tilt rotor and corresponding lift vector, thrust vector, and drag.

FIG. 4 is a diagram illustrating an embodiment of airflow produced when trailing edge mounted tilt rotors on a main wing are off.

FIG. 5A is a diagram illustrating an embodiment of a forward swept and tapered wing and a straight wing for comparison.

FIG. 5B is a diagram illustrating an embodiment of a wing configuration with a forward swept, tapered main wing and no canard.

FIG. 5C is a diagram illustrating an embodiment of a wing configuration with a canard and a straight main wing.

FIG. 5D is a diagram illustrating an embodiment of a tail.

FIG. 5E is a diagram illustrating an embodiment of a pylon which is attached to the top surface of the main wing.

FIG. 6A is a diagram illustrating an embodiment of a takeoff tilt change from hover position to cruise position.

FIG. 6B is a diagram illustrating an embodiment of a landing tilt change from cruise position to hover position.

FIG. 7 is a diagram illustrating an embodiment of a velocity tilt diagram.

FIG. 8A is a top view of an aircraft embodiment with a three-airfoil tail.

FIG. 8B is a front view of an aircraft embodiment with a three-airfoil tail.

FIG. 8C is a side view of an aircraft embodiment with a three-airfoil tail.

FIG. 9A is an angled view of an aircraft embodiment with a V tail.

FIG. 9B is a front view of an aircraft embodiment with a V tail.

FIG. 10 is a diagram illustrating an embodiment of a multicopter with a truncated fuselage which is capable of flying in a magic carpet mode.

FIG. 11A is a top view of a multicopter embodiment with a truncated fuselage and tail.

FIG. 11B is a side view of a multicopter embodiment with a truncated fuselage and tail.

DETAILED DESCRIPTION

The invention can be implemented in numerous ways, including as a process; an apparatus; a system; a composition of matter; a computer program product embodied on a computer readable storage medium; and/or a processor, such as a processor configured to execute instructions stored on and/or provided by a memory coupled to the processor. In this specification, these implementations, or any other form that the invention may take, may be referred to as techniques. In general, the order of the steps of disclosed processes may be altered within the scope of the invention. Unless stated otherwise, a component such as a processor or a memory described as being configured to perform a task may be implemented as a general component that is temporarily configured to perform the task at a given time or a specific component that is manufactured to perform the task. As used herein, the term ‘processor’ refers to one or more devices, circuits, and/or processing cores configured to process data, such as computer program instructions.

A detailed description of one or more embodiments of the invention is provided below along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment. The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material that is known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.

Various embodiments of an aircraft with a fixed wing to which rotors are attached on the trailing edge are described herein. In some embodiments, the aircraft includes a main wing, where the main wing is a fixed wing, and a main wing rotor that extends outward on a trailing edge side of the main wing where the aircraft is kept at least partially airborne at least some of the time by aerodynamic lift acting on the main wing, and the aircraft is kept at least partially airborne at least some of the time by airflow produced by the main wing rotor. In one example, the aircraft takes off vertically using the downward thrust from the rotor(s) (e.g., where there is almost no useful lift contribution from the wings) and then transitions to a mostly forward direction of flight (e.g., where all or almost all of the lift to keep the aircraft airborne comes from the wings). In some embodiments, there is always some combination from both sources (e.g., anything from 10% of lift from wing(s) and 90% of lift from rotor(s) to 90% of lift from wing(s) and 10% of lift from rotor(s), or any combination in between). In some embodiments, the main wing rotor is a tilt rotor that is able to rotate between a first rotor position and a second rotor position. In some embodiments, the main wing is a forward swept wing and/or is tapered. In some embodiments, there is also a canard which may include rotors (e.g., which may or may not tilt). The following figures illustrate various embodiments of the aircraft and as will be described in more detail below, various aircraft features enable the aircraft to improve upon previous aircraft designs.

FIG. 1 is a diagram illustrating a top view of a forward swept, fixed wing multicopter embodiment with tilt rotors. In the example shown, the main wing (100) is a fixed wing which is attached to the fuselage (102) in a fixed manner or position. The main wing is not, in other words, a tilt wing which is capable of rotating. The main wing (100) is also forward swept (e.g., relative to the pitch axis). For example, the forward-sweep angle may be on the order of θ_(sweep) between 14° and 16° for aircraft embodiments with a canard (as shown here) or as high as 35° for aircraft embodiments without a canard.

In this example, the main wing (100) has six rotors (104) which are attached to the trailing edge of the main wing. For clarity, these rotors are sometimes referred to as the main wing rotors (e.g., to differentiate them from the rotors which are attached to the canard). Naturally, the number of rotors shown here is merely exemplary and is not intended to be limiting.

In addition to the six main wing rotors, there are two rotors (106) which are attached to the canard (108). These rotors are sometimes referred to as the canard rotors. The canard is thinner than the main wing, so unlike the main wing rotors, the canard rotors are attached to the distal ends of the canard as opposed to the trailing edge of the canard.

All of the rotors in this example are tilt rotors, meaning that they are capable of tilting or otherwise rotating between two positions. In this example, the rotors on the left-hand (i.e., port) side of the aircraft are in a cruise (e.g., forward flight, backward facing, etc.) position. See, for example, the position of canard rotor 106. In this position, the rotors are rotating about the (e.g., substantially) longitudinal axes of rotation so that they provide (substantially) backward thrust. When the rotors are in this position, the lift to keep the multicopter airborne comes from the airflow over the main wing (100) and the canard (108). In various embodiments, the rotational range of a tilt rotor may be as low as 5 degrees or as high as 95 degrees and is design and/or implementation specific.

The rotors on the right-hand (i.e., starboard) side of the aircraft are in a hover (e.g., vertical takeoff and landing, downward facing, etc.) position. See, for example, the position of main wing rotor 104. In this second position, the rotors are rotating about (e.g., substantially) vertical axes of rotation so that they provide (substantially) downward thrust. In this configuration, the lift to keep the multicopter airborne comes from the downward airflow of the rotors.

Generally speaking, the tilt rotors, when oriented to output thrust substantially downward, permit the aircraft to perform vertical takeoff and landings (VTOL). This mode or configuration (e.g., with respect to the manner in which the aircraft as a whole is flown and/or with respect to the position of the tilt rotors specifically) is sometimes referred to as hovering. The ability to perform vertical takeoffs and landings permits the aircraft to take off and land in areas where there are no airports and/or runways. Once airborne, the tilt rotors (if desired) change position to output thrust (substantially) backwards instead of downwards. This permits the aircraft to fly in a manner that is more efficient for forward flight; this mode or configuration is sometimes referred to as cruising.

A canard is useful because it can stall first (e.g., before the main wing), creating a lot of pitching moment and not much loss of lift at stall whereas a main wing stall loses a lot of lift per change in pitching moment (e.g., causing the entire aircraft to drop or fall). Stalls are thus potentially more benign with a canard compared to without a canard. The canard stall behavior is particularly beneficial in combination with a swept forward wing, as the stall of the main wing can create an adverse pitching moment if at the wing root and can create large and dangerous rolling moments if at the wing tip. Furthermore, a canard can create lift at low airspeeds and increase CL_(max) (i.e., maximum lift coefficient) and provides a strut to hold or otherwise attach the canard motors to.

In some embodiments, the pylons (110) which are used to attach the rotors to the canard and/or main wing include some hinge and/or rotating mechanism so that the tilt rotors can rotate between the two positions shown. Any appropriate hinge mechanism may be used. For example, with ultralight aircraft, there are very stringent weight requirements and so a lightweight solution may be desirable. Alternatively, a fixed-tilt solution may also be used to meet very stringent weight requirements (as will be described in more detail below).

In some embodiments, the aircraft is designed so that the main wing (100) and canard (108) are able to provide sufficient lift to perform a glider-like landing if needed during an emergency. For example, some ultralight standards or specifications require the ability to land safely if one or more rotors fail and the ability to perform a glider-like landing would satisfy that requirement. One benefit to using a fixed wing for the main wing (e.g., as opposed to a tilt wing) is that there is no danger of the wing being stuck in the wrong position (e.g., a hover position) where a glider-like landing is not possible because of the wing position which is unsuitable for a glider-like landing.

Another benefit to a fixed wing with trailing edge mounted tilt rotors is stall behavior (or lack thereof) during a transition from hover position to cruise position or vice versa. With a tilt wing, during transition, the tilt wing's angle of attack changes which makes stalling an increased risk. A fixed wing with trailing edge mounted tilt rotors does not change the wing angle of attack (e.g., even if rotors are turned off/on or the tilt rotors are shifted). Also, this configuration both adds dynamic pressure and circulation over the main wing, which substantially improves the behavior during a transition (e.g., from hover position to cruise position or vice versa). In other words, the transition can be performed faster and/or more efficiently with a fixed wing with trailing edge mounted tilt rotors compared to a tilt wing (as an example).

Another benefit associated with tilt rotors (e.g., as opposed to a tilt wing) is that a smaller mass fraction is used for the tilt actuator(s). That is, multiple actuators for multiple tilt rotors (still) comprise a smaller mass fraction than a single, heavy actuator for a tilt wing. There are also fewer points of failure with tilt rotors since there are multiple actuators as opposed to a single (and heavy) actuator for the tilt wing. Another benefit is that a fixed wing makes the transition (e.g., between a cruising mode or position and a hovering mode or position) more stable and/or faster compared to a tilt wing design.

In some embodiments (not shown here), the rotors are variable pitch propellers which have different blade pitches when the rotors are in the hovering position versus the cruising position. For example, different (ranges of) blade pitches may enable more efficient operation or flight when in the cruise position (see, e.g., rotor 106) versus the hovering position (see, e.g., rotor 104). When the rotors are in a cruise position (see, e.g., rotor 106), putting the blade pitches into “cruising pitch” (e.g., on the order of 26°) enables low frontal area which is good for cruising (e.g., lower drag). When the rotors are in a hovering position (see, e.g., rotor 104), putting the blade pitches into a “hovering pitch” (e.g., on the order of 6°) enables high disc area which is good for hovering. To put it another way, one blade pitch may be well suited for cruising mode but not for hovering mode and vice versa. The use of variable pitch propellers enables better (e.g., overall) efficiency, resulting in less power consumption and/or increased flight range.

The following figures illustrate various benefits associated with the exemplary aircraft shown in FIG. 1.

FIG. 2A is a diagram illustrating a bottom view of an embodiment of boundary layer thicknesses with the motors off. In this example, laminar run lines 200 a, 202 a, and 204 a illustrate laminar runs at various regions of the main wing. In this example, it is assumed that the aircraft is cruising (e.g., flying in a substantially forward direction). As in FIG. 1, the main wing rotors (206) are attached to the trailing edge of the main wing (208) in this embodiment. The next figure shows the boundary layer thicknesses with the rotors turned on.

FIG. 2B is a diagram illustrating a bottom view of an embodiment of boundary layer thicknesses with motors on. In this example, the motors are on and the rotors have an exit airflow velocity of 30 m/s. With the motors on, a low pressure region is created towards the aft of the wing which increases the laminar run on the main wing. See, for example, laminar run lines 200 b, 202 b, and 204 b which correspond to laminar run lines 200 a, 202 a, and 204 a from FIG. 2A. A comparison of the two sets illustrates that the laminar runs have increased for the first two locations (i.e., at 200 a/200 b and 202 a/202 b). The last location (i.e., 204 a/204 b) has only a slightly longer laminar run length due to interference from the canard rotors (210).

The drag from the main wing rotors (more specifically, the drag from the pylons which are used to attach the main wing rotors to the main wing) is hidden in the wake of the airflow coming off the main wing. See, for example FIG. 2A which more clearly shows that the pylons (220) are connected or otherwise attached behind most of the extent of laminar run (222). With the embodiment shown here, the pylons also get to keep some of the boundary layer thickness from the main wing, which means the pylons have lower drag per surface area. This improves the drag compared to some other alternate designs or configurations. The following figures describe this in more detail.

FIG. 3A is a diagram illustrating an example of a tilt wing configuration with corresponding lift vector, thrust vector, and drag. In this example, a fixed rotor (300) is attached to a tilt wing (302) at a fixed position or angle. This is one alternate arrangement to the aircraft embodiment(s) described above. To direct the airflow produced by the fixed rotor (300) either backwards or downwards, the tilt wing (302) is rotated. As shown here, with this configuration, there is drag (304) at the trailing edge of the tilt wing, which is undesirable.

The lift (306) and thrust (308) for this configuration are also shown here, where the tilt wing is shown in the middle of a transition (e.g., between a cruising position and a hovering position). As shown here, the lift (306) and thrust (308) are substantially orthogonal to each other, which is inefficient. In other words, a tilt wing is inefficient during its transition.

FIG. 3B is a diagram illustrating an example of a fixed wing configuration with a leading edge mounted tilt rotor and corresponding lift vector, thrust vector, and drag. In this example, a tilt rotor (320) is attached to the leading edge of a fixed wing (322). This is another alternate arrangement to the aircraft embodiment(s) described above. The corresponding drag (324) and thrust (326) for this arrangement are also shown. There is no useful lift produced with this configuration and therefore no lift vector is shown here.

FIG. 3C is a diagram illustrating an embodiment of a fixed wing configuration with a trailing edge mounted tilt rotor and corresponding lift vector, thrust vector, and drag. In this example, the tilt rotor (340) is attached to the trailing edge of the fixed wing (342). In this configuration, the drag due to the trailing edge mounted tilt rotor (e.g., mostly due to its pylon, not shown) is hidden in the wake of the airflow coming off the main wing. As such, there is no drag (at least due to the tilt rotor (340)).

The position of the trailing edge mounted tilt rotor (340) relative to the fixed wing (342) also sucks air (344) over the fixed wing, after which the air turns or bends through the rotor and downwards. This flow turning over the wing generates a relatively large induced lift (346) which is shown here. The thrust vector (348) due to the rotors is also shown here. It is noted that the induced lift (346) and thrust (348) are substantially in the same direction (i.e., both are pointing substantially upwards) which is a more efficient arrangement, including during a transition. In other words, using a fixed wing with trailing edge mounted tilt rotors produces less drag and improved efficiency during a transition (e.g., due to the lift and thrust vectors which now point in substantially the same direction) compared to other rotor and wing arrangements. Note for example, drag 304 and drag 324 in FIG. 3A and FIG. 3B, respectively and the orthogonal positions of lift 306 and thrust 308 in FIG. 3A.

The following figure illustrates an embodiment of flow turning in more detail.

FIG. 4 is a diagram illustrating an embodiment of airflow produced when trailing edge mounted tilt rotors on a main wing are off. In this example, a tilt rotor multicopter (400) is shown but with the main wing rotors turned off for comparison purposes. With the rotors off, the airflow in (402) and the airflow out (404) are moving in substantially the same direction. That is, the airflow does not turn (e.g., downwards) as it passes through the rotors.

Multicopter 420 shows the same multicopter as multicopter 400 except the rotors are turned on. In this example, the airflow in (422) and the airflow out (424) have noticeable different directions and there is noticeable turning or bending of the airflow as it passes through the rotors of the exemplary multicopter shown. As described above, this induces a noticeable lift, which is desirable because less power is consumed and/or the range of the multicopter increases.

In this example, the main wing rotors (426) are in the hovering position. As shown here, these rotors are slightly pitched or otherwise angled (e.g., with the tops of the main wing rotors pointing slightly forward and the bottoms pointing slightly backward). In this diagram, the amount of tilting is shown as θ_(pitch) (428) and in some embodiments is on the order of 90° of rotational range or movement (e.g., ˜3° up from horizontal when in a cruise position (e.g., for minimum drag) and ˜93° degrees down from horizontal when in a hover position which produces a rotational range of ˜96°). Although this angling or pitching of the rotors is not absolutely necessary for flow turning to occur, in some embodiments the main wing rotors are angled or otherwise pitched to some degree in order to increase or otherwise optimize the amount of flow turning. In some embodiments, the canard rotors are similarly pitched. It is noted that multicopter 420 is shown in a nose up position and therefore the vertical axis (e.g., relative to the multicopter) is not perpendicular to the ground and/or frame of reference.

In some embodiments, the rotors (e.g., the main wing rotors and/or canard rotors) are rolled or otherwise angled slightly outward, away from the fuselage, when the rotors are in hovering position. In some embodiments, this roll (e.g., outward) is on the order of 10° for greater yaw authority.

In some embodiments, the main wing is tapered (e.g., the wing narrows going outward towards the tip) in addition to being forward swept. The following figures describe various wing and/or tail embodiments.

FIG. 5A is a diagram illustrating an embodiment of a forward swept and tapered wing and a straight wing for comparison. In the example shown, wing 500 is a straight wing with no tapering (e.g., the wing is the same width from the center to the tip of the wing). Exemplary rotors (502) are shown at the trailing edge of the string wing (500).

The center of thrust (504), indicated by a dashed and dotted line, is dictated by the placement or arrangement of the rotors and runs through the centers of the main wing rotors (502). For simplicity, the canard rotors are ignored in this example. The center of lift is based on the shape of the wing. For a rectangular wing such as wing 500, the center of lift (506), indicated by a solid line, runs down the center of the wing. Calculation of the aerodynamic center is more complicated (e.g., the aerodynamic center depends upon the cross section of the wing, etc.) and aerodynamic center 508, indicated by a dashed line, is exemplary and/or typical for this type of wing.

As shown here, the straight wing (500) and its corresponding arrangement of main wing rotors (502) produce a center of thrust (504) which is relatively far from both the center of lift (506) as well as the aerodynamic center. This separation is undesirable. More specifically, when the main wing rotors (502) are in hover position, if the center of thrust (504) is far from the center of lift (506), then the transition (e.g., in the context of the movement of the aircraft as a whole, such as switching from flying substantially upwards to substantially forwards or vice versa) would create very large moments and could overturn the vehicle or prevent acceleration or stability and/or or require a massive and/or non-optimal propulsion system. In cruise, if the center of thrust (504) is far from center of lift (506), it not as important (e.g., since the thrust moments are both smaller and more easily balanced by aerodynamic moments), but it is still undesirable.

In contrast, the forward swept and tapered wing (520) and its corresponding arrangement of rotors (522) along the trailing edge produce a center of thrust (524), center of lift (526), and aerodynamic center (528) which are closer to each other. For example, the forward sweep of the wing brings the rotors forward to varying degrees. This causes the center of thrust to move forward (e.g., towards the leading edge and towards the other centers). The tapering of the wings prevents the aerodynamic center and center of lift from creeping forward too much (and more importantly, away from the center of thrust) as a result of the forward sweep. For example, with a forward swept wing with no tapering (not shown), the center of thrust would move forward approximately the same amount as the aerodynamic center and center of lift and would result in more separation between the three centers than is shown here with wing 520.

Some other benefits to a forward swept and tapered wing include better pilot visibility, and a better fuselage junction location with the main wing (e.g., so that the main wing spar can pass behind the pilot seat, not through the pilot). Furthermore, the taper reduces wing moments and puts the center of the thrust of the motors closer to the wing attachment to the fuselage, as referenced about the direction of flight, so there are less moments carried from wing to fuselage, a shorter tail boom (e.g., which reduces the weight of the aircraft), and improved pitch stability.

FIG. 5B is a diagram illustrating an embodiment of a wing configuration with a forward swept, tapered main wing and no canard. In this example, the main wing (530) is forward swept and tapers (e.g., from the center of the wing to the tip). The tips (532) are rounded where the trailing edge follows the shape or contour of the outermost rotor (534) when that rotor is in a hover position as shown here. This may be attractive from a safety point of view because having the main wing wrap around the front of the rotor (e.g., with a relatively small gap between the blades of the rotor and the trailing edge of the main wing), enables the main wing to act as a shield for at least the front side of the outermost rotor (534). In this example, there are 10 main wing rotors (e.g., including rotor 534) which are attached to the trailing edge of the main wing (530).

In this wing configuration there is no canard. To compensate for the lack of a canard and/or canard rotors, the main wing has more forward sweep than if there was a canard and/or canard rotors. For example, the θ_(sweep) of the leading edge or spar shown here may be on the order of 20° or 30° as opposed to on the order of 10°-15° when there is a canard and/or canard rotors.

This type of wing configuration is attractive in applications where sensor placement or other volumetric or structural requirement makes it infeasible to attach a canard to the fuselage forebody area. It also has the fringe benefit of providing additional protection to the pilot compartment in case of a blade-out (e.g., a blade shatters and/or becomes a projectile) even on one of the propellers, since the main wing blocks a substantial portion of the blade's trajectory cone intersecting the cockpit. It also may be beneficial in terms of simplicity and a reduction in the number of components on the system, and can be useful with a different number of rotors on the vehicle, where for packaging reasons a canard is not sensible.

FIG. 5C is a diagram illustrating an embodiment of a wing configuration with a canard and a straight main wing. In this example, there is a canard (540) with four canard rotors (542) attached to the trailing edge of the canard. There is also a main wing (544) which is straight with six main wing rotors (546) attached to the trailing edge of the main wing. The center of lift, center of thrust, and aerodynamic center (now shown) may be relatively close to each other with the center of lift and center of thrust in front of the aerodynamic center, all of which are desirable properties or characteristics.

This type of wing configuration is attractive in applications where wing sweep is unfavorable structurally or from a controls standpoint, or where a compact vehicle footprint is required while increasing the available lift, and where induced drag is not important. The additional canard area helps with additional lift availability in forward flight and transition while additional rotors help with an increase in lift during hover. Adding additional rotors to the canard instead of the main wing allows the center of thrust to move forward, matching the forward motion of the aerodynamic center due to increase in canard area, provided the canard has a small area Increasing canard area allows vehicle span to remain unchanged or smaller compared to increasing lifting surface area by scaling up a large main wing.

FIG. 5D is a diagram illustrating an embodiment of a tail. In some embodiments, an aircraft includes a tail (i.e., a tail is not necessary) and this diagram shown one example of a tail. In this example, the tail (550) has two control surfaces (552) such as flaps. A control rotor (554) is attached the trailing edge of the tail at the center of that edge. As shown here, the control rotor may be oriented so that the pushes air downward. In various embodiments, the control rotor (554) a fixed rotor or a tilt rotor. In some embodiments, if the control rotor is a tilt rotor, there would be no leading edge rotors (556). The tail also includes two tail rotors (556) which are attached to the leading edge of the tail. In some embodiments, the leading edge rotors (556) are fixed rotors.

FIG. 5E is a diagram illustrating an embodiment of a pylon which is attached to the top surface of the main wing. There are a variety of ways to attach the main wing rotors to the main wing and this is merely one example. In this example, the main wing rotors (560 a/560 b) are attached to a pylon (562). The pylon, in turn, in attached to the top surface of the main wing (564) where there is a gap (566) between the pylon and the main wing. As a result, there is a duct-like effect at gap 566 between the pylon (560) and the main wing (564). In addition, offsetting the pylons from the wing's upper surface leads to additional lift and lower drag on the wing surface, at the expense of increased drag on the pylon skin.

As shown here, the back of the pylon (560) where the rotor (560 a/560 b) extends beyond the back of the main wing (564). This permits sufficient clearance for the rotor to rotate without hitting the main wing when in cruise position (560 a), in hover position (560 b), or in any position in between the two extremes.

As described above, in some embodiments, the canard rotors (if there are any) and main wing rotors are tilt rotors and the rotors are able to switch (if desired) between two positions for more efficient flight. (A corollary to this is that slow flight (e.g., below stall speed for a traditional fixed wing) may be maintained by varying the degree of tilt rather than tilting only between the two extreme or terminal positions.) The following figures describe exemplary tilt transitions of the rotors between cruise position and hover position.

FIG. 6A is a diagram illustrating an embodiment of a takeoff tilt change from hover position to cruise position. In some embodiments, the exemplary multicopter performs this transition soon after taking off (e.g., substantially vertically). It is noted that this tilt transition is optional and the aircraft may fly entirely with the rotors in the hovering position (albeit with less than optimal performance). For example, this could be done if there is risk in the tilting action, and it would be better to take the action at a higher altitude.

Multicopter 600 shows the exemplary aircraft after it has performed a vertical takeoff. In this state shown here, the main wing rotors and canard rotors are in hover position (e.g., rotating about a substantially vertical axis of rotation so that the rotors generate substantially downward thrust).

The multicopter then transitions from an entirely upward direction of movement to a direction of movement with at least some forward motion with the rotors remaining in the hover position until the multicopter reaches some desired altitude at which to begin the transition (602). In other words, the vehicle transitions first, and then changes the tilt of the rotors. In one example, the altitude at which the multicopter begins the rotor tilt change from hover position to cruise position is an altitude which is sufficiently high enough for there to be recovery time in case something goes wrong during the transition. Switching the rotors between hover position and cruise position is a riskier time where the likelihood of something going wrong (e.g., a rotor failing, a rotor getting stuck, etc.) is higher. Although the multicopter may have systems and/or techniques in place for recovery (e.g., compensating for a rotor being out by having the remaining rotors output more thrust, deploy a parachute, etc.), these systems and/or techniques take time (i.e., sufficient altitude) to work.

From position 602, the multicopter flies substantially forward and moves the tilt rotors from a hover position (e.g., where thrust is output substantially downward) to a cruise position. Once in the cruise position 604, the rotors rotate about a substantially longitudinal axis so that they output backward thrust.

FIG. 6B is a diagram illustrating an embodiment of a landing tilt change from cruise position to hover position. For example, the exemplary multicopter may perform this transition before landing vertically. As with the previous transition, this transition is optional. For example, the exemplary multicopter can keep the tilt rotors in cruise position and perform a glider-like landing as opposed to a vertical landing if desired.

Multicopter 610 shows the rotors in a cruise position. While flying in a substantially forward direction, the tilt rotors are moved from the cruise position shown at 610 to the hover position shown at 612. With the tilt rotors in the hover position (612), the multicopter descends with some forward movement (at least in this example) so as to keep power use low(er) and retain better options in the case of a failure of a motor or other component (e.g., the multicopter can power up the rotors and pull out of the landing process or path) to position 614 until it finally lands on the ground.

FIG. 7 is a diagram illustrating an embodiment of a velocity tilt diagram. In the diagram shown, the x-axis shows the forward speed of the aircraft and the y-axis shows the tilt (e.g., position or angle of the tilt wing or tilt rotors) which ranges from a (e.g., minimal) cruise position (700) to a (e.g., maximal) hover position (702).

The first operating envelope (704), shown with a solid border and filled with a grid pattern, is associated with a tilt wing aircraft. See, for example, multicopter 400 in FIG. 4 and tilt wing 302 and fixed rotor 300 in FIG. 3A. The second operating envelope (706), shown with a dashed border and gray fill, is associated with a (e.g., comparable) aircraft with a forward swept and fixed wing with trailing edge mounted tilt rotors. See, for example, the embodiments described above.

In the diagram shown here, the tilt rotor operating envelope (706) is a superset of the tilt wing operating envelope (704) which indicates that the former aircraft configuration is safer and/or more airworthy than the latter and is also able to fly both faster and slower at comparable tilt positions. With a fixed wing, the wing is already (and/or always) pointed in the direction of (forward) travel. When the tilt rotors are at or near the (e.g., maximal) hover position (702), the vehicle can fly around pretty much all the way up to the stall speed (e.g., V₂) without having to tilt the motors up to cruise position. Note, for example, that the tilt rotor operating envelope (706) can stay at the (e.g., maximal) hover position (702) all the way up to V₂. This greatly increases the operating regime of the tilt rotor operating envelope (706) compared to the tilt wing operating envelope (704). Note for example, all of the gray area above the tilt wing operating envelope (704).

Another effect which can contribute to the expanded operating envelope for the tilt rotor configuration at or near hover position includes flow turning (see, e.g., FIG. 4). The flow turning over the main wing induces some extra lift. In some embodiments, this flow turning and its resulting lift are amplified or optimized by tilting the main wing rotors at a slight backward angle from directly down when in a normal hover (e.g., at minimal tilt position 700).

In contrast, a tilt wing presents a large frontal area when the tilt wing is tilted up in (e.g., maximal) hover position (702). As a result, tilt wings are unable to fly forward at any kind of decent speed until at or near the full (e.g., minimal) cruise position (700) or nearly so.

The following figures illustrate more detailed embodiments of a forward swept, fixed wing aircraft with tilt rotors, including some optional features not described above.

FIG. 8A is a top view of an aircraft embodiment with a three-airfoil tail. In the example shown, the exemplary multicopter includes a tail (800) with three airfoils: two horizontal stabilizers (802) and a single vertical stabilizer (804). The tips (806) of the main wing in this example are curved. For example, this may help to better capture forced airflow from the tip of the propellers, as well as adding to the wing aspect ratio. This results in lower induced drag and higher available lift for a given power input. An additional benefit is protecting tip propeller blades from lateral strikes while conducting hover operations close to structures. The main wing also includes a shoulder (808) which widens the part of the main wing which connects to the fuselage and helps structurally.

FIG. 8B is a front view of an aircraft embodiment with a three-airfoil tail. FIG. 8B continues the example of FIG. 8A. As shown here, the canard rotors (820) are positioned so that they are below the (plane of the) main wing (822). This positioning of the canard rotors improves the thrust line in cruise and reduces interaction between the canard rotors (820) and the main wing (822).

FIG. 8C is a side view of an aircraft embodiment with a three-airfoil tail. As this view shows, the canard rotors (840) are positioned so that they are below the (plane of the) main wing (842).

FIG. 9A is an angled view of an aircraft embodiment with a V tail. In this example, the multicopter has a V tail (900). One benefit to a V tail (900) is that it helps to avoid interactions between the main wing rotors (902) and the surfaces of the tail (900). The following figure shows a front view which more clearly illustrates this.

FIG. 9B is a front view of an aircraft embodiment with a V tail. FIG. 9B continues the example of FIG. 9A. As shown from this view, the V tail (910) rises above the main wing rotors as it extends outward from the fuselage to the tip of the wing. As a result, even when the main wing rotors are in cruising position (912), the V tail (910) is not directly behind and/or directly in the wake of the main wing rotors (912). This minimizes the interactions between the main wing rotors (912) and the V tail's surfaces (910).

This view also shows that (in this embodiment at least) the main wing rotors in hover (914) are angled or otherwise tilted slightly backward and slightly outward. As described above, this may be desirable because desirable because it allows at least some aircraft embodiments to fly in a “magic carpet mode” where the rotors are still in a hover tilt position, but can transition to primarily wing borne flight. The following figure shows an example of this.

FIG. 10 is a diagram illustrating an embodiment of a multicopter with a truncated fuselage which is capable of flying in a magic carpet mode. As used herein, the term magic carpet mode refers to a mode in which the rotors are still in a hovering orientation, but the vehicle has been accelerated to an airspeed where a substantial amount of lift is generated by the wing. In the magic carpet mode, the vehicle speed can be controlled with forward pitch, and altitude can be controlled either by increasing speed to gain efficiency and thus climb rate, or by directly adding thrust to the rotors. In the example shown, the multicopter has a canard (1000) with two canard rotors (1002). The main wing (1004), which is a fixed wing with a forward sweep, has six main wing rotors (1006) which are attached to the trailing edge of the main wing. The fuselage (1008) is relatively short and is referred to herein as a truncated fuselage. For example, note that the end of the fuselage (1010) extends only a little bit past the end of the backmost rotor (1012). In this particular embodiment, the rotors are fixed and do not tilt or otherwise change position.

There are a variety of multicopter embodiments which are capable of meeting stringent weight requirements (e.g., an ultralight standard). In this approach, the truncated fuselage is much shorter and there is no tail per se, both of which keep the weight down. The use of fixed rotors (e.g., as opposed to tilt rotors) also keeps the weight down. The truncated fuselage and lack of a tail also produces a smaller footprint which helps with transport (e.g., in a trailer) and the amount of space required for takeoff and/or landing.

In some embodiments, the rotors at a fixed position tilted back, more on the hover end of the tilt spectrum as opposed to the cruise end of the tilt spectrum (e.g., an axis of rotation that is tilted downward from horizontal at an angle between 20° to 40°, inclusive). See, for example the axis of rotation (1020) associated with fixed rotor (1022) where the tilt angle is between 20° to 40° which suitable and/or acceptable for magic carpet mode. For example, this rotor position (although fixed) permits the exemplary multicopter to fly vertically (e.g., not due to aerodynamic lift on the wing, but from the airflow produced by the rotors) as well as forwards (e.g., off the wing). This ability or mode of keeping the rotors in a hover-style tilt while flying (e.g., primarily and/or mostly) in a wing borne manner is sometimes referred to as a fly magic carpet mode. It is noted that this ability to fly in a magic carpet mode is not necessarily limited to fixed rotor embodiments. For example, some or all of the above tilt rotor embodiments may be flown in magic carpet mode (e.g., where the tilt position is the extreme or maximal hover position, or some tilt position between the two extremes).

FIG. 11A is a top view of a multicopter embodiment with a truncated fuselage and tail. The embodiment shown here has similarities with the previous multicopter embodiment shown in FIG. 10 and for brevity shared features are not discussed herein. Unlike the previous example, this embodiment has a tail (1100). The fuselage (1102) is a truncated fuselage so the tail (1100) and fuselage (1102) are connected using a boom (1104).

FIG. 11B is a side view of a multicopter embodiment with a truncated fuselage and tail. FIG. 11B continues the example of FIG. 11A. From this view, other features of the multicopter, including a horizontal control surface (1106) and a vertical control surface (1108) on the tail and ski-like landing gear (1110) are more clearly shown.

Although the foregoing embodiments have been described in some detail for purposes of clarity of understanding, the invention is not limited to the details provided. There are many alternative ways of implementing the invention. The disclosed embodiments are illustrative and not restrictive. 

What is claimed is:
 1. An aircraft, comprising: a fuselage; a fixed and forward-swept wing; a plurality of port-side tilt rotors, wherein the plurality of port-side tilt rotors is coupled to a trailing edge of the fixed and forward-swept wing on the port side of the aircraft; and a plurality of starboard-side tilt rotors, wherein the plurality of starboard-side tilt rotors is coupled to the trailing edge of the fixed and forward-swept wing on the starboard side of the aircraft, wherein: the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are configured to rotate between a first rotor position and a second rotor position; when the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are in the first rotor position, at least some of a lift to keep the aircraft airborne comes from thrust output by the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors at least some of the time; and when the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are in the second rotor position, at least some of the lift to keep the aircraft airborne comes from aerodynamic lift on the fixed and forward-swept wing at least some of the time.
 2. The aircraft recited in claim 1, wherein the fixed and forward-swept wing includes a tapered wing.
 3. The aircraft recited in claim 1 further comprising one or more of the following: a V tail or a three-airfoil tail.
 4. The aircraft recited in claim 1 further comprising: a canard; and a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position.
 5. The aircraft recited in claim 1 further comprising: a canard; a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position; the canard tilt rotor is attached to the canard at a first height; and the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are attached to the fixed and forward-swept wing at a second height that is higher than the first height.
 6. The aircraft recited in claim 1 further comprising: a canard; a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position; the canard tilt rotor is attached to the canard at a first height; and the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are attached to the fixed and forward-swept wing at a second height that is higher than the first is height, wherein the fixed and forward-swept wing includes a tapered wing.
 7. The aircraft recited in claim 1 further comprising: a canard; a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position; the canard tilt rotor is attached to the canard at a first height; the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are attached to the fixed and forward-swept wing at a second height that is higher than the first height; and a V tail.
 8. The aircraft recited in claim 1 further comprising: a canard; a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position; the canard tilt rotor is attached to the canard at a first height; the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are attached to the fixed and forward-swept wing at a second height that is higher than the first height; and a three-airfoil tail.
 9. A method, comprising: rotating a plurality of port-side tilt rotors and a plurality of starboard-side tilt rotors into a first rotor position, wherein: when the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are in the first rotor position, at least some of a lift to keep an aircraft airborne comes from thrust output by the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors at least some of the time; and the aircraft includes: a fuselage; a fixed and forward-swept wing; the plurality of port-side tilt rotors, wherein the plurality of port-side tilt rotors is coupled to a trailing edge of the fixed and forward-swept wing on the port side of the aircraft; the plurality of starboard-side tilt rotors, wherein the plurality of starboard-side tilt rotors is coupled to the trailing edge of the fixed and forward-swept wing on the starboard side of the aircraft; and the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are configured to rotate between the first rotor position and a second rotor position; and rotating the plurality of port-side tilt rotors and a plurality of starboard-side tilt rotors into the second rotor position, wherein when the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are in the second rotor position, at least some of the lift to keep the aircraft airborne comes from aerodynamic lift on the fixed and forward-swept wing at least some of the time.
 10. The method recited in claim 9, wherein the fixed and forward-swept wing includes a tapered wing.
 11. The method recited in claim 9, wherein the aircraft further includes one or more of the following: a V tail or a three-airfoil tail.
 12. The method recited in claim 9, wherein the aircraft further includes: a canard; and a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position.
 13. The method recited in claim 9, wherein the aircraft further includes: a canard; a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position; the canard tilt rotor is attached to the canard at a first height; and the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are attached to the fixed and forward-swept wing at a second height that is higher than the first height.
 14. The method recited in claim 9, wherein the aircraft further includes: a canard; a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position; the canard tilt rotor is attached to the canard at a first height; and the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are attached to the fixed and forward-swept wing at a second height that is higher than the first height, wherein the fixed and forward-swept wing includes a tapered wing.
 15. The method recited in claim 9, wherein the aircraft further includes: a canard; a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position; the canard tilt rotor is attached to the canard at a first height; the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are attached to the fixed and forward-swept wing at a second height that is higher than the first height; and a V tail.
 16. The method recited in claim 9, wherein the aircraft further includes: a canard; a canard tilt rotor that is attached to the canard, wherein the canard tilt rotor is configured to rotate between a third rotor position and a fourth rotor position; the canard tilt rotor is attached to the canard at a first height; the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are attached to the fixed and forward-swept wing at a second height that is higher than the first height; and a three-airfoil tail.
 17. A computer program product, the computer program product being embodied in a non-transitory computer readable storage medium and comprising computer instructions for: rotating a plurality of port-side tilt rotors and a plurality of starboard-side tilt rotors into a first rotor position, wherein: when the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are in the first rotor position, at least some of a lift to keep an aircraft airborne comes from thrust output by the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors at least some of the time; and the aircraft includes: a fuselage; a fixed and forward-swept wing; the plurality of port-side tilt rotors, wherein the plurality of port-side tilt rotors is coupled to a trailing edge of the fixed and forward-swept wing on the port side of the aircraft; the plurality of starboard-side tilt rotors, wherein the plurality of starboard-side tilt rotors is coupled to the trailing edge of the fixed and forward-swept wing on the starboard side of the aircraft; and the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are configured to rotate between the first rotor position and a second rotor position; and rotating the plurality of port-side tilt rotors and a plurality of starboard-side tilt rotors into the second rotor position, wherein when the plurality of port-side tilt rotors and the plurality of starboard-side tilt rotors are in the second rotor position, at least some of the lift to keep the aircraft airborne comes from aerodynamic lift on the fixed and forward-swept wing at least some of the time. 